Laser System Detects Explosives Remotely

Traditionally, the most sensitive techniques for detecting explosives have involved sampling and analyzing the air near a suspected bomb. This is time-consuming and potentially dangerous. Now a team at Soreq Nuclear Research Center in Yavne, Israel, has developed a fast, remote method: a laser that pulls double duty.

"Both steps are induced by the same, single laser pulse," explained Talya Arusi-Parpar, a researcher on the project. "The process itself is divided in two: first the dissociation and then the detection." Specifically, the laser first dissociates any trace trinitrotoluene vapor in the air into its various constituents, some of which are vibrationally excited nitric oxide radicals. These NO fragments then may be detected by their laser-induced fluorescence.

The approach has two advantages in terms of sensitivity. First, the wavelength of the fluorescence is lower than the wavelength of the exciting laser, which is in contrast to the fluorescence of other molecules in the air. Second, there is no background fluorescence from ground- state NO.

In a series of experiments, the researchers used a Spectra-Physics Nd:YAG laser to generate a 248-nm beam with a 10-Hz repetition rate, which they sent through the sample. They positioned spectral filters in front of a Hamamatsu photomultiplier tube at a 90° angle and analyzed its output.

Using this setup, the team detected TNT concentrations of less than 8 ppb at atmospheric pressure and room temperature. This compares favorably to the sensitivity of other methods, such as well-trained sniffer dogs.

Toward 10-m applications

In the future, the laser technique could be extended to detect other explosives with nitrogen groups, but the sensitivity will depend on the vapor pressure of the explosive in question. In addition, the method could remotely screen baggage and packages, a possibility that Arusi-Parpar and others are exploring.

"We are working on a system capable of detecting nitro-containing explosive vapor at distances of 10 m," she said. Remote detection at 2.5 m has already been achieved, and a compact laser source for 10-m applications is being developed.

Dov Heflinger and Raphael Lavi helped develop the method, which benefited from the collaboration of Ilana Bar's team at Ben-Gurion University of the Negev in Beersheba, Israel, and which received funding from the US and Israeli governments.